Use of Glucose and Acetate by Bacteria and Algae in Aquatic Ecosystems

New methods employing C 1 4—Labeled organic compounds measure the uptake of organic solutes by planktonic microorganisms. By testing uptake over a wide range of substrate concentrations, two separate mechanisms can be differentiated and their kinetics measured. Using filtration and evidence from experiments with laboratory cultures of planktonic bacteria and algae, specific transport systems effective at very low substrate concentrations were traced to the bacteria, and a diffusion mechanism, effective only at higher substrate concentrations, to the algae. Studies have shown that V, the maximum velocity of uptake by bacterial transport systems, gives information about the size and function of the bacterial populations. A diffusion constant, k d , gives information on the rate of uptake of solutes by the algal populations. Turnover times, derived from kinetic parameters, indicate that the algal uptake of glucose and acetate in Lake Erken, Sweden, is always less than 10% of the bacterial uptake, even though the algal biomass may be orders of magnitude greater than the bacterial. Two new types of bioassay employ the kinetics of bacterial uptake systems as the measuring reaction. Acetate and glucose were found in 1—10 mg/liter concentrations in the several natural waters tested. At these very low concentrations, algal uptake of glucose and acetate is so low that effective heterotrophy is impossible. In contrast, the bacteria effectively remove substrate from solution at these low levels and probably keep the substrate at these low concentrations. By doing this, the bacteria may prevent heterotrophic growth of algae in nature.